Feeling the (pollen tube) force

If you think about a pollen grain that, after all of that bee-hitchhiking, has landed on the sweet stigma of the right flower, you might think that it has finally arrived. But from the pollen grain’s point of view its active struggle has just started – it now has to penetrate through the stigma tissue to localise and fertilise the ovule.

Most of the research carried out on this part of the pollen grain’s journey has focused on the chemical cues that lure the pollen tube in the direction of the ovule, but as Jan Burri and his co-authors from the University of Zurich and ETH Zurich point out in their new paper “a major hurdle for the PT (pollen tube) on its way to the ovule is the physical interaction with the surrounding tissues”.

Image: System configuration to simulate the natural environment of the pollen tube (PT) in vitro with integrated force readout. (a) The experimental system combines a lab‐on‐a‐chip (LOC) device with a microelectromechanical systems (MEMS)‐based force sensor. (b) An optical image of a LOC loaded with pollen grains in the inner reservoir; the PTs grow in parallel in the same focal plane through the microchannels. (c) Capacitive force sensor measuring lateral forces at the force‐sensing plate. (d) Schematic side‐view of the experimental setup where the force‐sensing plate is placed in front of the PTs emerging from the microchannels.
System configuration to simulate the natural environment of the pollen tube (PT) in vitro with integrated force readout. (a) The experimental system combines a lab‐on‐a‐chip (LOC) device with a microelectromechanical systems (MEMS)‐based force sensor. (b) An optical image of a LOC loaded with pollen grains in the inner reservoir; the PTs grow in parallel in the same focal plane through the microchannels. (c) Capacitive force sensor measuring lateral forces at the force‐sensing plate. (d) Schematic side‐view of the experimental setup where the force‐sensing plate is placed in front of the PTs emerging from the microchannels.

To measure the forces produced by the pollen tube while it is squeezing its way through the tiny spaces between cells, the researchers set up a new system – the Lab-on-a-Chip, which is made by a series of parallel channels in which the pollen tubes can grow straight, with a micro force sensor placed a few micrometers from the channels’ exit.

After initial contact with the mechanical sensor, the pollen tubes continue to grow in a straight direction, pushing on the sensor, until the force reaches a certain threshold. At that point it stops for a few seconds and then it starts growing again, but this time in a different direction, avoiding the obstacle. The force at which the pollen tube perceives the mechanical force and responds varies between the two species involved in this study (Easter Lily and Arabidopsis, a monocot and a dicot), but the general pattern of the response is the same.

But avoiding an obstacle is not all that is required from the strenuous pollen, as it then needs to squeeze through a narrow opening between the Chip and the sensor, which mimics the pollen tube’s journey as it penetrates the spaces between cells in the transmitting tract of the stigma. To do so it locates the gap, anchors in it, and then grows inside, causing a steep increase in the force applied to the sensor and slightly widening the narrow space.

The scientists suggest that as this new method allows them to simultaneously observe and measure pollen responses, it will help to investigate the roles of mechanical cues at the single-cell level. It could also be used to study the pathways involved in the response, and to discover new ones, perhaps using fluorescent molecules.

At the moment, when the pollen tube finally emerges from the gap between the Lab-on-a-Chip and the micromechanical sensor, it finds no ovule waiting, but maybe someday it will.

Read the paper: Burri, J. T., Vogler, H. , Läubli, N. F., Hu, C. , Grossniklaus, U. and Nelson, B. J. (2018) Feeling the force: how pollen tubes deal with obstacles. New Phytologist. doi: 10.1111/nph.15260

Zoe Nemec Venza
Research Collaborator
New Phytologist Trust

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Zoe Nemec Venza

Zoe Nemec Venza is a Sainsbury PhD student at the University of Bristol, in Dr Jill Harrison’s lab, funded by the Gatsby Charitable Foundation. She graduated from her Masters degree at the Universita’ di Pisa, with Prof. Francesco Licausi. Her research interests are mostly located in the broad field of plant development. She is particularly fascinated by how cell identity is established and by how developmental pathways changed during evolution. Zoe is doing an internship at the New Phytologist Trust during summer 2018.